1. Field of the Invention
The present invention relates to a method of laser exposure applied to an image recording apparatus such as a laser printer, a color scanner for reproduction, or a laser plotter for manufacturing printed circuit boards, and more specifically to a method of laser exposure in which a plurality of parallel laser recording beams scan an image recording surface.
2. Description of the Prior Art
FIG. 1 is a perspective view of a conventional laser plotter and FIG. 2 is a schematic block diagram of such laser plotter.
Referring to FIGS. 1 and 2, the laser plotter comprises a console unit 21, a data processing unit 23 for arithmetically processing data inputted through a recording medium such as a magnetic tape, a data converting unit 70 for converting the data processed in the data processing unit 23 into desired recording signals, and a recording unit 80 for scanning and recording images on a photosensitive film 83 on a recording drum 82 based on the recording signals. The plotter exposes and records a master pattern of high precision of a printed circuit board and the like at high speed and with high precision.
The laser plotter can be functionally divided into an image data forming unit 60, a data converting unit 70 and an image recording unit 80. The image data forming unit 60 forms vector data of the image based on CAD (Computer Aided Design) data. The image data forming unit comprises a minicomputer 61 for calculating vector data, a CRT 62, a keyboard 63, a magnetic tape 64 or magnetic disc 65 for storing CAD data, and the like. The data converting unit 70 converts the vector data formed in the image data forming unit to dot data for output. The image recording unit 80 records a binary image by zonally scanning a photosensitive material (which will be the image recording surface) with a plurality of multilaser beams, whether or not a particular beam is scanned on the recording surface is based on a plurality of dot data supplied from the data converting unit 70. The image recording unit 80 comprises a laser unit for emitting recording a laser, a recording drum 82 which is rotated with the photosensitive material 83 held thereon, a multichannel type modulator for separately outputting multiple laser beams, and the like.
In the above described apparatus, simultaneous recording is carried out by several beams, as follows: Beam spots are arranged adjacent to each other in an axial direction (subscanning direction) of the recording drum 82, whereby an image is formed in the axial direction. After the image is formed in the axial direction, the recording drum 82 is rotated in the peripheral direction (main scanning direction). The beams are formed in the subscanning direction based on the succeeding data. The above described operation is repeated to record the image.
However, if recording laser beams emitted from a single laser source overlap with each other, the recording beams are disturbed because of interference. Therefore, the prior art has sought to avoid overlapping the recording beams.
The distribution of light intensity of a laser beam emitted from a laser source is uneven. The intensity has a Gaussian distribution represented by the following equation (1), as shown in FIG. 3, for example. EQU I.sub.y =I.sub.0 exp [-2(y/.omega.).sup.2 ] (1)
where
I.sub.y : power density PA1 y: distance from the center of the beam PA1 .omega.: beam radius (where the power density becomes 1/e.sup.2 in association with the power density I.sub.0 at the center)
As is apparent from the equation (1), the intensity of light becomes extremely low at the periphery of the beam.
If the laser beam has a Gaussian distribution, the beam spot diameter D (=2.omega.) is defined as a beam diameter having the light intensity distribution of 1/e.sup.2 (about 13.5%) of the light intensity at the center of the laser beam.
The overlap of the beams is defined as the overlap of the beam spot diameters D.
Therefore, if the zonal scanning lines are successively exposed and recorded by a plurality of adjacent such recording beams, the density of the portion corresponding to the border between the two beams cannot be high and the resulting recording pattern will appear to be divided and will not have the desired even density.
Methods for eliminating such a disadvantage are disclosed in, for example, Japanese Patent Laying-Open Gazette No. 118302/1977, Japanese Patent Laying-Open Gazette No. 69701/1978, Japanese Patent Laying-Open Gazette No. 203071/1985.
FIG. 4 shows a method disclosed therein. A serial beam spot line B13 is arranged inclined by a prescribed angle from the scanning direction, so that portions of neighboring scanning lines overlap each other as they scan the photosensitive material with this method, the above described phenomenon in which the image appears divided can be eliminated.
However, since the serial beam spot line is arranged inclined from the scanning direction by a prescribed angle in the above described prior art, if the image signal output timings of respective recording beams are made equal, the resulting image is skewed according to the angle of inclination. Therefore, a delay circuit is necessary to control the timing of image output signals for each recording beam so that the resulting image is correctly aligned. The number of required delay circuits corresponds to the number of recording beams. This complicates the apparatus.
Furthermore, when the image recording surface is on a cylindrical surface of a rotary recording drum whose, the direction of the recording beam line B13 is diagonal to the axial direction of the recording drum 25 as is shown exaggerated in FIG. 4. Therefore, the shape of the beam (beam spot B14 of each recording beam) irradiated onto the cylindrical recording surface is distorted. Consequently, the light intensity distribution becomes uneven.
Another method for eliminating the above described disadvantages is disclosed in Japanese Patent Laying-Open Gazette No. 203071/1985.
A plurality of laser recording beams are transmitted through glass fibers and the beam output ends of the glass fibers are arranged in a staggered manner so as to form a staggered beam spot line on the image recording surface, whereby portions of neighboring scanning lines overlap each other.
However, when a plurality of laser beams are transmitted through glass fibers, the above described prior art creates the following problems.
Generally, a beam emitted from the beam output end of a glass fiber has a large angle of divergence (25.degree..about.120.degree.) (very large relative to the angle of divergence of a common laser beam which is about 1 m rad). Thus, the loss of energy of a recording beam emitted from a glass fiber is great. In order to eliminate this problem, a collimator lens may be provided at the beam output end of each glass fiber. However, this makes the structure quite complicated and presents other practical difficulties and disadvantages.